The fleet of 22 Deltics, numbered from D9000 to D9021, entered traffic during 1961-62 on the East Coast Main Line, replacing the 55 steam locos – including 35 ‘A4′ Class locos such as Mallard, which had previously operated on the route. The locos were a development of the prototype DELTIC, which had been built by English Electric a few years earlier and was trialled on both the East Coast and West Coast Main Lines between 1955 and 1960, covering more than 450,000 miles.
 

Based at Finsbury Park (London), Gateshead (Newcastle) and Haymarket (Edinburgh), the Deltics were the most powerful diesel locomotives on British Railways and were powered by two Napier Deltic D18-25B power units rated at 1650 hp each. With a design speed of 105 mph, they were also the fastest locomotives at the time of construction. The first example to emerge from Vulcan Foundry at Newton-le-Willows, where the locos were constructed, was D9001, on January 16th, 1961. It was followed later in the month by D9000, delivery of this having been delayed owing to the fitting of a flashing light on the nose. Deliveries were completed in March 1962, and three months later a major revision of the East Coast Main Line timetable took place, which saw three services between King’s Cross and Edinburgh and vice-versa booked to undertake the journey in under six hours for the first time.
 

Outshopped from English Electric’s Vulcan Foundry in attractive two-tone green, this soon gave way to corporate BR blue, the first Deltic to be so treated being D9002 The Kings Own Yorkshire Light Infantry in 1966. During 1967 and 1968, the locos were fitted with air brakes (they had been vacuum braked only initially). Another major modification was the fitting of ETH (Electric Train Heating) equipment in 1970/71, prior to the introduction of air-conditioned coaching stock on the East Coast Main Line. The locos retained their vacuum brakes and steam heat boilers.

The introduction of InterCity 125 (HST) trains on the ECML in 1978/79 resulted in the Deltics no longer being required for many of the services between London and Scotland, and as a result, the locos based at Haymarket and Gateshead were transferred to York in May 1979. From then on, the locos were used on ‘semi-fast’ services between Kings Cross and York and started to put in appearances on the Trans-Pennine route between York and Liverpool.
 

The first withdrawals (those of D9001 St Paddy and D9020 Nimbus) took place in early 1980, both locos having stood out of use at Doncaster Works since 1978. They were both cut up within a few weeks of withdrawal. As more HSTs entered traffic over the next two years, the fleet was gradually run down, with D9003 Meld being taken out of service at the end of 1980 and D9005/6 in February 1981. The last BR passenger workings took place on December 31st, 1981 (when D9015/17/19/21 were all out in passenger service), with a final railtour running from Kings Cross to Edinburgh two days later, the event making the national news.
 

D9002 was claimed for the National Collection and moved straight to the National Railway Museum in York, while the remaining locos were taken to Doncaster Works for disposal. Most were cut up, but five examples were saved. The DPS saved D9009 Alycidon and D9019 Royal Highland Fusilier, while D9022 Royal Scots Grey and D9016 Gordon Highlander were purchased by the Deltic 9000 Fund. D9015 Tulyar was put up for auction at Christies in December 1982, but failed to reach its reserve price. It was bought by Peter Sansom in 1984, and then purchased by the DPS in 1986.